1. Field of the Invention
The present invention relates to a hydraulic system of a backhoe in which a swiveling base provided with a ground work apparatus is mounted so as to be capable of swiveling around a vertical axis on a traveling body.
2. Description of the Related Art
JP 2006-161510A discloses a hydraulic system of a backhoe in which a swiveling base provided with a hydraulically-driven ground work apparatus is mounted so as to be capable of swiveling around a vertical axis on a traveling body provided with a hydraulically-driven left-right pair of travel apparatuses. In this backhoe hydraulic system, during a state of non-travel, pressurized oil from a first pump and a second pump merges together and is supplied to the ground work apparatus, and pressurized oil from a third pump is supplied to a swiveling motor that allows the swiveling base to swivel. During travel, pressurized oil from the first pump is supplied to one of the left and right travel apparatuses, and pressurized oil from the second pump is supplied to the other of the left and right travel apparatuses, so that pressurized oil from the first pump and pressurized oil from the second pump is supplied independently, and pressurized oil from the third pump is supplied to a hydraulic actuator of the ground work apparatus.
In this conventional hydraulic system, a first channel switching valve and a second channel switching valve are provided. The first channel switching valve is switchable between a merging position where pressurized oil from the first pump and the second pump merges together and is supplied to a ground work apparatus control valve, and an independent supply position where pressurized oil from the first pump and the second pump is respectively independently supplied to a control valve for the left and right travel apparatuses. The second channel switching valve is switchable between a non-supply position where pressurized oil from the third pump is not supplied to the ground work apparatus control valve, and a supply position where pressurized oil from the third pump is supplied to the ground work apparatus control valve. Also, in this conventional hydraulic system, a travel detection circuit is provided that is in communication with a discharge oil path of a fourth pump via an orifice for introducing pressurized oil, and detects that the travel apparatus control valve has been operated. The travel detection circuit is configured to detect that the travel apparatus control valve has been operated when pressure is established in the circuit due to part of the circuit being blocked when the travel apparatus control valve has been operated.
The first channel switching valve and the second channel switching valve are configured with a pilot operation switching valve that is switched by a pilot pressure. With these valves, pilot pressure established in the travel detection circuit when the travel apparatus control valve has been operated is fed to both the first channel switching valve and the second channel switching valve, and when detected that the ground work apparatus control valve has been operated, the pilot pressure is fed to the second channel switching valve. The first channel switching valve is switched from the merging position to the independent supply position by the pilot pressure established in the travel detection circuit due to the travel apparatus control valve being operated. The second channel switching valve, during a state of non-travel remains in the non-supply position without being switched to the supply position by the pilot pressure established due to the ground work apparatus control valve being operated. Furthermore, when the ground work apparatus is being used and the travel apparatus control valve has been operated, the second channel switching valve is switched to the supply position by the sum pilot pressure of the pilot pressure established due to the ground work apparatus control valve being operated and the pilot pressure established in the travel detection circuit due to the travel apparatus control valve being operated.
In this sort of hydraulic system, a phenomenon occurs that in a case where the travel apparatus control valve has been operated while the ground work apparatus is in use, when the first channel switching valve is switched before the second channel switching valve, for example, when a travel operation has been performed while a boom was being raised, supply of pressurized oil to a boom cylinder that allows the boom to be operated is temporarily interrupted, so boom operation is temporarily stopped. Therefore, it is necessary to adopt settings such that when the travel apparatus control valve has been operated while the ground work apparatus is in use, the second channel switching valve is switched at the same time as the first channel switching valve, or the second channel switching valve is switched before the first channel switching valve.
Also, in the above conventional hydraulic system, pressurized oil from the high volume first and second pumps passes through the first channel switching valve, so the diameter of a spool of the first channel switching valve is comparatively large relative to the second channel switching valve or the like in order to suppress loss of pressure. Also, the first channel is switched by the pilot pressure established in the travel detection circuit that is in communication with the discharge oil path of the fourth pump via the orifice, so in order to improve the response of switching of the first channel switching valve when the travel apparatus control valve has been operated, it is necessary to enlarge the diameter of the orifice for introduction of pressurized oil to the travel detection circuit (on the upstream side of the travel detection circuit), so that a large amount of pressurized oil is introduced to the travel detection circuit from the fourth pump.
When the diameter of the orifice for introduction of pressurized oil to the travel detection circuit is enlarged, the neutral pressure of the travel detection circuit (circuit pressure of the travel detection circuit in a state in which part of the travel detection circuit is not blocked) when at low temperature increases, and thus the first channel switching valve becomes sensitive. Also, when the neutral pressure of the travel detection circuit is high, there is less freedom for setting the switching pressure of the first channel switching valve.
On the other hand, when the second channel switching valve is switched to the supply position by the sum pilot pressure of the pilot pressure established due to the ground work apparatus control valve being operated and the pilot pressure established in the travel detection circuit due to the travel apparatus control valve being operated, and the ground work apparatus is being operated, the second channel switching valve cannot be switched to the supply position for various reasons even though the travel apparatus control valve is not being operated, so the switching pressure of the second channel switching valve cannot be set very low in order to eliminate such a circumstance from occurring (in order that the second channel switching valve is reliably switched to the supply position when the ground work apparatus control valve and the travel apparatus control valve have been operated).
Also, response will worsen if the switching pressure of the first channel switching valve is greatly increased, and in any event there is a limit to how much the switching pressure can be increased.
For the above reasons, in the circuit configuration of a conventional hydraulic system, when attempting to satisfy both improving the response of switching of the first channel switching valve when the travel apparatus control valve has been operated, and insuring reliability of switching of the second channel switching valve, it is difficult to adjust the switching pressure of the first channel switching valve and the second channel switching valve such that the second channel switching valve is switched at the same time as the first channel switching valve or before the first channel switching valve, and therefore there are instances when the first channel switching valve is switched before the second switching valve when the travel apparatus control valve has been operated while the ground work apparatus is in use.